Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention. The development of secondary batteries capable of repeatedly charging and discharging has been the focus of particular interest. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Many secondary batteries are currently available. Lithium secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and much higher energy densities than conventional batteries using aqueous electrolyte solutions, such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire or explosion, encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. However, additional research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, safety regulations strictly prohibit the dangers (such as fire and smoke) of electrochemical devices. In connection with the safety characteristics of a lithium secondary battery including a separator, overheating of the lithium secondary battery may cause thermal runaway or puncture of the separator may pose an increased risk of explosion. In particular, a porous polyolefin substrate commonly used as a separator of a lithium secondary battery undergoes extreme thermal shrinkage at a temperature of 100° C. or higher due to its material characteristics and production processes including elongation. This thermal shrinkage behavior may cause short circuits between a cathode and an anode.
Various proposals have been made to solve the above safety problems of electrochemical devices. For example, Korean Unexamined Patent Publication No. 10-2007-231 discloses a separator which includes a porous organic-inorganic coating layer formed by coating a mixture of inorganic particles and a binder polymer on at least one surface of a porous substrate. The inorganic particles present in the porous organic-inorganic coating layer coated on the porous substrate serve as spacers that can maintain a physical shape of the porous organic-inorganic coating layer to inhibit the porous substrate from thermal shrinkage when an electrochemical device overheats. Cavities present between the inorganic particles form fine pores of the porous coating layer.
The inorganic particles should be present in an amount above a predetermined level in order to allow the porous organic-inorganic coating layer to inhibit thermal shrinkage of the porous substrate. However, as the content of the inorganic particles increases (that is, as the content of the binder polymer decreases), the bindability of the separator to an electrode may deteriorate and the inorganic particles may be separated from the porous organic-inorganic coating layer when stress occurs during fabrication (e.g., winding) of an electrochemical device by assembly. The separated inorganic particles act as local defects of the electrochemical device, giving a negative influence on the safety of the electrochemical device. Thus, there is a need to develop a binder polymer that can enhance the bindability of a porous organic-inorganic coating layer to a porous substrate. There is also a need to improve the bindability of a porous organic-inorganic coating layer to an electrode.
On the other hand, a porous organic-inorganic coating layer having a low packing density should be formed to a large thickness so as to perform its functions. This acts as an obstacle to the production of a thin separator essential to enhance the capacity of an electrochemical device.